Data quality report - gaps in XBT data

In nearly all series there are occurrences where there are large depth differences between adjacent cycles. These large gaps occur at intermittent intervals, with size and frequency varying from cast to cast. In the majority of series, the magnitude of the depth differences ranges from a couple of metres to approximately 25 m. However, there are a few XBT casts which have gaps larger than these. In these cases of larger depth differences, a separate data quality report was generated for each cast, highlighting the location and magnitude of the depth gap between cycles.

Although the Data Originator has not provided an explanation for these gaps, there are no indications that they make the data untrustworthy. Comparisons performed by the Originator between the XBT data and nearby CTD and minilogger data revealed close matches between temperature measurements for these three sensors.

Data Originator comments on the quality of D318 XBT data

In some profiles recorded using the XBT-T5 sensor, the maximum depth of the instrument was increased by the Originator above the manufacturer recommended depth, to a new value of 2200 m. Although there was no apparent loss of data quality at this increased depth, the Originator warns that at the bottom of the profile, temperature starts to increase for an undetermined reason. As this was not seen in the CTD profiles performed during this cruise, the Data Originator warns that these data are likely not representative of true ocean structures.

The Data Originator warns that in some of the early XBT casts, the instrument started to measure temperature values before submersion. This modified the true water surface temperature and consequently the depth values. Due to this, the Originator only considers the data to be of good quality after some seconds, when the temperature values start to decrease with depth.

Open Data supplied by Natural Environment Research Council (NERC)

Instrument description

Lockheed Martin Sippican T-5 XBT Probe

The Expendable Bathythermograph system uses a sea water ground. As soon as an electrode within the nose of the expendable probe makes contact with the water, the circuit is complete and temperature or sound velocity data can be telemetered to the ship-board data processing equipment. The T-5 XBT Probe can be used within a maximum depth of 1830 m, with a rated ship speed of 6 knots and has a vertical resolution of 65 cm.

Originator's processing document for D318 XCTD and XBT data

Sampling strategy

A total of 32 Lockhead Martin Sippican expendable conductivity, temperature and depth (XCTD) casts and 515 Lockhead Martin Sippican expendable bathythermograph (XBT) casts were conducted during RSS Discovery cruise D318, which took place in the Gulf of Cadiz (for more information see the D318 cruise report ). The cruise was split up into two legs, D318a and D318b. D318a took place from 17 April 2007 to 23 April 2007, with D318b taking place from 27 April 2007 to 14 May 2007. Of the 32 XCTD casts, 22 were performed using XCTD-1 probes with the remaining 10 performed by XCTD-2 instruments. 496 of the 515 XBT casts were performed using XBT-T5 units with the remaining 19 being performed by XBT-T7 probes.

Deployment

Casts were performed at intervals of approximately 20 minutes. For the first two days every fifth cast was performed by an XCTD, however the use of XCTD probes became more sporadic from 20 April 2007, after the originator noticed spiking in the XCTD-2 data. This resulted in long periods of the cruise where only XBT-T5 instruments were used. The use of XCTDs became even more infrequent during leg D318b, with only 10 XCTD casts being performed.

During D318a, XBT and XCTD deployments occurred simultaneously with the deployment of a towed seismic array (air guns and hydrophone streamer). To combat the possibility of the XBT being entangled in the array the data originator rigged a guide pipe to keep the XBT wire further out on the starboard side of the RSS Discovery. This pipe comprised of around five metres of rubberised tubing with a diameter of approximately six centimetres, which was taped to a metal bar to keep it straight. One end was lashed to the rear starboard side of the ship with the other held outwards by the rear starboard crane at an angle of approximately 45° The lower end of the pipe was approximately three metres starboard of the rear of the ship, one to two metres above the sea surface. When launched the probes fell down the tube and into the sea below, away from the line of the towed array. Although this arrangement worked very well for the most part, some of the longer XCTD probes got stuck and needed to be shaken out of the tube.

Data from the XBT/XCTD sensors was transmitted up a length of copper wire to the Sippican data acquisition system. At the beginning of the cruise the old Sippican ISA data acquisition system was used to communicate with the XBTs and XCTDs, however this system started to pre-trigger the instruments before they hit the water. This led to the scientists switching to the USB system, which remained stable throughout the cruise.

Data processing

The originator processed the XBT and XCTD data for the following:

1) Determination of depth

XBTs and XCTDs measure depth as a proxy of elapsed descent time through the water column (this is a known variable). For D318, the XCTD depths were generated using the standard Sippican conversion equation. The Originator reports however, that for the XBT data the Sippican equation has been found to underestimate depth. They resolved this by using a more accurate equation proposed by Boyd and Linzell (1993) for the T5 and T7 XBTs. The originator reports that a temperature correction from Boyd and Linzell (1993) has also been applied to the XBT data. Additionally, the Originator derived sound velocity in both the XBT and XCTD data. For the XCTDs measured temperature and salinity data were used to compute sound velocity. For the XBTs, sound velocity was derived from the measured temperature data with an assumed constant salinity of 35 PSU.

2) Removal of bad data

The Data Originator reports that spiking was a persistent problem in some of the XCTD casts, rendering some portions of these data too troublesome to use. This was rectified by removing all data that the XCTD processing software flagged as bad data (those not given an 8000 flag by the Sippican software). Additionally, all XBTs which displayed persistent trends or peaks were also excluded from the data sent to BODC.

Calibrations

All XCTDs were purchased specifically for this cruise. Prior to transmission the data was calibrated using internally stored calibration coefficients calculated at three different temperatures and conductivities.

References

Processing of D318 XBT data by BODC

The Sippican XBT data were supplied to BODC in the form of 498 ASCII files. Following standard BODC procedure, the data files were reformatted into BODC internal format using an internal transfer function. This table shows how the variables present in the XBT data files were mapped to appropriate BODC parameter codes.

Originator's variable

Description

Units

BODC parameter code

Units

Depth

Depth below surface of the water body by computation from probe free-fall time using unspecified algorithm

m

DEPHCV01

m

Temperature

Temperature of the water body by expendable bathythermograph (XBT)

°C

TEMPET01

°C

Sound velocity

The rate at which sound travels through the water column

m s -1

SVELXXXX

m s -1

The reformatted data were visualised using the in-house EDSERPLO software. Suspect data were marked by adding an appropriate quality control flag, missing data by both setting the data to an appropriate value and setting the quality control flag.

General Data Screening carried out by BODC

BODC screen both the series header qualifying information and the parameter values in the data cycles themselves.

Header information is inspected for:

Irregularities such as unfeasible values

Inconsistencies between related information, for example:

Times for instrument deployment and for start/end of data series

Length of record and the number of data cycles/cycle interval

Parameters expected and the parameters actually present in the data cycles

Originator's comments on meter/mooring performance and data quality

Documents are written by BODC highlighting irregularities which cannot be resolved.

Data cycles are inspected using time or depth series plots of all parameters. Currents are additionally inspected using vector scatter plots and time series plots of North and East velocity components. These presentations undergo intrinsic and extrinsic screening to detect infeasible values within the data cycles themselves and inconsistencies as seen when comparing characteristics of adjacent data sets displaced with respect to depth, position or time. Values suspected of being of non-oceanographic origin may be tagged with the BODC flag denoting suspect value; the data values will not be altered.

The following types of irregularity, each relying on visual detection in the plot, are amongst those which may be flagged as suspect:

If a large percentage of the data is affected by irregularities then a Problem Report will be written rather than flagging the individual suspect values. Problem Reports are also used to highlight irregularities seen in the graphical data presentations.

Inconsistencies between the characteristics of the data set and those of its neighbours are sought and, where necessary, documented. This covers inconsistencies such as the following:

Maximum and minimum values of parameters (spikes excluded).

The occurrence of meteorological events.

This intrinsic and extrinsic screening of the parameter values seeks to confirm the qualifying information and the source laboratory's comments on the series. In screening and collating information, every care is taken to ensure that errors of BODC making are not introduced.

The Natural Environment Research Council (NERC) funds the Oceans 2025 programme, which was originally planned in the context of NERC's 2002-2007 strategy and later realigned to NERC's subsequent strategy (Next Generation Science for Planet Earth; NERC 2007).

Who is involved in the programme?

The Oceans 2025 programme was designed by and is to be implemented through seven leading UK marine centres. The marine centres work together in coordination and are also supported by cooperation and input from government bodies, universities and other partners. The seven marine centres are:

National Oceanography Centre, Southampton (NOCS)

Plymouth Marine Laboratory (PML)

Marine Biological Association (MBA)

Sir Alister Hardy Foundation for Marine Science (SAHFOS)

Proudman Oceanographic Laboratory (POL)

Scottish Association for Marine Science (SAMS)

Sea Mammal Research Unit (SMRU)

Oceans2025 provides funding to three national marine facilities, which provide services to the wider UK marine community, in addition to the Oceans 2025 community. These facilities are:

British Oceanographic Data Centre (BODC), hosted at POL

Permanent Service for Mean Sea Level (PSMSL), hosted at POL

Culture Collection of Algae and Protozoa (CCAP), hosted at SAMS

The NERC-run Strategic Ocean Funding Initiative (SOFI) provides additional support to the programme by funding additional research projects and studentships that closely complement the Oceans 2025 programme, primarily through universities.

What is the programme about?

Oceans 2025 sets out to address some key challenges that face the UK as a result of a changing marine environment. The research funded through the programme sets out to increase understanding of the size, nature and impacts of these changes, with the aim to:

improve knowledge of how the seas behave, not just now but in the future;

help assess what that might mean for the Earth system and for society;

assist in developing sustainable solutions for the management of marine resources for future generations;

enhance the research capabilities and facilities available for UK marine science.

In order to address these aims there are nine science themes supported by the Oceans 2025 programme:

Climate, circulation and sea level (Theme 1)

Marine biogeochemical cycles (Theme 2)

Shelf and coastal processes (Theme 3)

Biodiversity and ecosystem functioning (Theme 4)

Continental margins and deep ocean (Theme 5)

Sustainable marine resources (Theme 6)

Technology development (Theme 8)

Next generation ocean prediction (Theme 9)

Integration of sustained observations in the marine environment (Theme 10)

In the original programme proposal there was a theme on health and human impacts (Theme 7). The elements of this Theme have subsequently been included in Themes 3 and 9.

When is the programme active?

The programme started in April 2007 with funding for 5 years.

Brief summary of the programme fieldwork/data

Programme fieldwork and data collection are to be achieved through:

physical, biological and chemical parameters sampling throughout the North and South Atlantic during collaborative research cruises aboard NERC's research vessels RRS Discovery, RRS James Cook and RRS James Clark Ross;

the Continuous Plankton Recorder being deployed by SAHFOS in the North Atlantic and North Pacific on 'ships of opportunity';

physical parameters measured and relayed in near real-time by fixed moorings and ARGO floats;

The data is to be fed into models for validation and future projections. Greater detail can be found in the Theme documents.

Oceans 2025 Theme 3: Shelf and Coastal Processes

Over the next 20 years, UK local marine environments are predicted to experience ever-increasing rates of change - including increased temperature and seawater acidity, changing freshwater run-off, changes in sea level, and a likely increase in flooding events - causing great concern for those charged with their management and protection. The future quality, health and sustainability of UK marine waters require improved appreciation of the complex interactions that occur not only within the coastal and shelf environment, but also between the environment and human actions. This knowledge must primarily be provided by whole-system operational numerical models, able to provide reliable predictions of short and long-term system responses to change.

However, such tools are only viable if scientists understand the underlying processes they are attempting to model and can interpret the resulting data. Many fundamental processes in shelf edge, shelf, coastal and estuarine systems, particularly across key interfaces in the environment, are not fully understood.

What are the consequences of these interactions on the functioning of the whole coastal system, including its sensitivity and/or resilience to change?

Ultimately, what changes should be expected to be seen in the UK coastal environment over the next 50 years and beyond and how might these changes be transmitted into the open ocean?

Within Oceans 2025, Theme 3 will develop the necessary understanding of interacting processes to enable the consequences of environmental and anthropogenic change on UK shelf seas, coasts and estuaries to be predicted. Theme 3 will also provide knowledge that can improve the forecasting capability of models being used for the operational management of human activities in the coastal marine environment. Theme 3 is therefore directly relevant to all three of NERC's current strategic priorities; Earth's Life-Support Systems, Climate Change, and Sustainable Economies

The official Oceans 2025 documentation for this Theme is available from the following link: Oceans 2025 Theme 3

Oceans 2025 Theme 3, Work Package 3.1: Global Impacts of Shelf Seas

At the margins of the shelf seas, steep shelf-slope bathymetry has impacts on ocean circulation and the transmission of signals around the ocean basins (Hughes and Meredith, 2006), while dense water formation and cascades at the shelf edge are thought to be important for water mass formation (Ivanov et al., 2004) and for the off-shelf transport of organic and inorganic carbon (e.g. Wollast and Chou, 2001).

In this Work Package, the Proudman Oceanographic Laboratory (POL) aim to quantify the water fluxes between the shelf and open ocean globally, including the development of methods to incorporate shelf effects into global models. Greater understanding of the whole carbon cycle will benefit from combining this work on down-slope fluxes of water (and its constituent dissolved carbon) with work in Oceans 2025 Theme 5 (down-slope transports of sediments and particulate carbon).

The specific objectives are:

Quantify and predict dense-water formation, cascading, slope mixing, their effects in the ocean

Determine constraints that the ocean margin imposes on adjacent ocean circulation and fields

Some data used in Work Package 3.1 were collected to complement work carried out on the European Union's Geophysical Oceanography (GO) project. For these data, linkage to the GO project documentation is provided